arXiv subjects

Nico Wagner

Publications and source records attributed to Nico Wagner.

6 recordsLinked to original sources

arXiv2026

Cordierite-based optical resonators with extremely low thermal expansion

Applications for ultra-stable lasers outside controlled laboratory environments require compact and robust optical resonators with reduced sensitivity to temperature fluctuations. The low thermal expansion coefficient (CTE) and the high stiffness make cordierite-based ceramics, such as NEXCERA, attractive for vibration insensitive room-temperature resonators. We revisit the effective CTE of resonators with spacers and mirrors made of different materials and use finite element simulations to analyze the impact of a CTE mismatch in a cordierite-based resonator with mirrors made of ultra-low expansion (ULE) glass or fused silica (FS). This enabled us to determine the CTE of a cordierite spacer from the measured effective CTE of a resonator. We confirm a six-fold larger CTE slope of cordierite around the zero-crossing temperature than in ULE glass. The steep CTE slope, in combination with the large stiffness, makes cordierite-based resonators far less sensitive to CTE mismatch with FS mirrors, thereby eliminating the need for additional compensation rings. We further consider the so far neglected case, where the CTE of the spacer is larger than that of the mirror, and propose resonator designs in which the thermal length change of the spacer is fully or partially compensated by the deflection of the mirrors. This results in a cordierite-based resonator with ULE mirrors whose effective CTE can be close to zero over a temperature range of several tens of Kelvin. We are extending our concept to resonators based on crystalline materials with high stiffness and low isothermal length change, such as silicon, enabling compact and robust room-temperature resonators for terrestrial and space-born applications.

physics.optics↗

arXiv2025

Quantum Doubly Stochastic Transformers

At the core of the Transformer, the softmax normalizes the attention matrix to be right stochastic. Previous research has shown that this often de-stabilizes training and that enforcing the attention matrix to be doubly stochastic (through Sinkhorn's algorithm) consistently improves performance across different tasks, domains and Transformer flavors. However, Sinkhorn's algorithm is iterative, approximative, non-parametric and thus inflexible w.r.t. the obtained doubly stochastic matrix (DSM). Recently, it has been proven that DSMs can be obtained with a parametric quantum circuit, yielding a novel quantum inductive bias for DSMs with no known classical analogue. Motivated by this, we demonstrate the feasibility of a hybrid classical-quantum doubly stochastic Transformer (QDSFormer) that replaces the softmax in the self-attention layer with a variational quantum circuit. We study the expressive power of the circuit and find that it yields more diverse DSMs that better preserve information than classical operators. Across multiple small-scale object recognition tasks, we find that our QDSFormer consistently surpasses both a standard ViT and other doubly stochastic Transformers. Beyond the Sinkformer, this comparison includes a novel quantum-inspired doubly stochastic Transformer (based on QR decomposition) that can be of independent interest. Our QDSFormer also shows improved training stability and lower performance variation suggesting that it may mitigate the notoriously unstable training of ViTs on small-scale data.

cs.LG↗

arXiv2025

Guidelines for designs for ultrastable laser with $\mathbf{10^{-17}}$ fractional frequency instability

Lasers with long coherence time and narrow linewidth are an essential tool for quantum sensors and clocks. Ultrastable cavities and laser systems are now commercially available with fractional frequency instabilities in the mid $10^{-16}$ range. This document aims to provide technical guidance for researchers starting in the field of ultrastable lasers and to give an outlook toward the next generation of improved ultrastable lasers. These guidelines have arisen from the scope of the EMPIR project ``Next generation ultrastable lasers'' ( https://www.ptb.de/empir2021/nextlasers ) with contributions from the European project partners.

physics.optics↗

arXiv2024

Black-box Uncertainty Quantification Method for LLM-as-a-Judge

LLM-as-a-Judge is a widely used method for evaluating the performance of Large Language Models (LLMs) across various tasks. We address the challenge of quantifying the uncertainty of LLM-as-a-Judge evaluations. While uncertainty quantification has been well-studied in other domains, applying it effectively to LLMs poses unique challenges due to their complex decision-making capabilities and computational demands. In this paper, we introduce a novel method for quantifying uncertainty designed to enhance the trustworthiness of LLM-as-a-Judge evaluations. The method quantifies uncertainty by analyzing the relationships between generated assessments and possible ratings. By cross-evaluating these relationships and constructing a confusion matrix based on token probabilities, the method derives labels of high or low uncertainty. We evaluate our method across multiple benchmarks, demonstrating a strong correlation between the accuracy of LLM evaluations and the derived uncertainty scores. Our findings suggest that this method can significantly improve the reliability and consistency of LLM-as-a-Judge evaluations.

cs.LG↗

arXiv2024

Temperature-dependent mechanical losses of Eu$^{3+}$:Y$_{2}$SiO$_{5}$ for spectral hole burning laser stabilization

We investigate the mechanical loss characteristics of Eu$^{3+}$:Y$_2$SiO$_5$$\unicode{x2013}$a promising candidate for ultra-low-noise frequency stabilization through the spectral hole burning technique. Three different mechanical oscillators with varying surface-to-volume ratios and crystal orientations are evaluated. In this context, we perform mechanical ringdown and spectral measurements spanning temperatures from room temperature down to $15\,\mathrm{K}$. By doing so, we measure a maximum mechanical quality factor of $Q=3676$, corresponding to a loss angle of $\phi=2.72\times 10^{-4}$. For a spectral hole burning laser stabilization experiment at $300\,\mathrm{mK}$, we can estimate the Allan deviation of the fractional frequency instability due to Brownian thermal noise to be below $\sigma_{\delta \nu/\nu_0} = 2.5\times 10^{-18}$, a value lower than the estimated thermal-noise limit of any current cavity-referenced ultra-stable laser experiment.

physics.optics↗

arXiv2024

Mechanical loss and stability analysis of NEXCERA in ultra-stable optical cavities

NEXCERA has emerged as a ceramic-based material for spacers in ultra-stable optical cavities, with a coefficient of thermal expansion that crosses zero near room temperature. In such cavities, frequency stability is ultimately limited by Brownian thermal noise in the cavity components. A key parameter in this context is the mechanical loss, which has remained unknown for NEXCERA. In this work, we investigate the mechanical loss of NEXCERA N117B at room temperature for various resonances using the gentle nodal suspension technique. We measure a promising minimum mechanical loss of $\phi = 1.89\times 10^{-5}$, indicating the suitability of NEXCERA for low-noise optical cavities. Using this value, we calculate the thermal noise of a cavity with a NEXCERA spacer and compare its performance to established materials such as ULE and Zerodur, taking into account different mirror substrate options. Our analysis shows that NEXCERA is a strong candidate for ultra-stable cavities due to its low thermal noise. Combined with its previously reported low linear drift, it offers a highly attractive option for long-term stable optical frequency references.

physics.optics↗